Easy to drive (low input gate capacitance) output transistor (yes I consider BIGBT as one joint part only), very stabile output transistor as integrity (balanced conditions for NPN/PNP), IRF conduct only one base current for both outputs, so for one base current (and it is the same for both outputs) through IRF driver you get two parallel collector curents for the output (doubled hFE). Linear transconductance dIout/dVg, high speed ... you should try it once.

R6, R7 are voltage dumpers to CCS, necessary to use because 2N j-fets have too low Vds/power dissipation specs. Current through R6, R7 is constant as CCS-s are capable of, so ideally no modulation should be present on these resistors. Of course it would be better to use source/sink transistors without voltage dumpers (double cascoded CCS). CCS are not ideal and there's always tendency to have increasing level of distortions according to higher frequencies. Separate measurements of CCS shows their capability to maintain the current to be constant. If you suggest to replace the resistors with improved CCS-s you are invited to do so.

Possible ripple suppression would be to add small parallel caps across R6, R7, just to prevent AC modulation (low level ripple) as voltage drop on them.

what if you reverse the positions.. so you place the resistors at the rails and the current sources where they belong. then you get both....reduced dissipation and no signal induced current modulation.